package db

import (
	"errors"

	"doormile/utils"

	nats "github.com/nats-io/nats.go"
)

// streamSubjects is the contract between this binary's js.Publish calls and the
// JetStream server. It lives here, next to the code that publishes, on purpose:
// the streams used to be created only by an external Python script
// (Birock/doormile-bookings/setup_jetstream.py) on another machine, and the two
// drifted. Four of the eight subjects this app published were bound to no stream
// at all — booking.cancelled, booking.outcome, booking.assignment_failed, and
// chat.room.closed.<id> (CHAT declared the literal "chat.room.closed", which does
// not match a subject with an extra token). Every publish site is best-effort
// (`if Js != nil` + warn-log), so those events were failing and being dropped
// silently rather than surfacing as an error anywhere.
//
// Adding a subject here is what makes a new js.Publish actually durable. If you
// add a publish call and skip this map, the event goes nowhere.
var streamSubjects = map[string][]string{
	"BOOKINGS": {
		"api.v1.bookings.create",
		"api.v1.bookings.update",
		"api.v1.bookings.cancel",
	},
	"ASSIGNMENTS": {
		"booking.assigned",
		"booking.reassigned",
		"booking.assignment_failed",
	},
	"STATUS": {
		"booking.status.updated",
		"booking.cancelled",
		"booking.outcome",
	},
	"TRACKING": {
		"miler.location.updated",
		"miler.stalled",
	},
	"NOTIFICATIONS": {
		"notification.send",
	},
	"CHAT": {
		"chat.room.created",
		"chat.room.closed",
		// chat.go publishes chat.room.closed.<bookingid>; the bare subject above
		// does not match it. Both are kept because the bare one may already have
		// messages bound to it from the previous configuration.
		"chat.room.closed.*",
	},
}

// EnsureStreams declares the streams above, idempotently, at startup.
//
// It only ever *adds*: a stream that already exists keeps its storage type,
// retention, limits and every subject it already had, and is updated only when
// this map names a subject it is missing. Nothing is deleted and no existing
// subject is dropped, so running this against the streams the Python script
// created is safe and converges rather than clobbering.
//
// Failures are logged and never fatal — NATS is best-effort everywhere in this
// codebase, and the API must still serve requests when the event bus is down.
func EnsureStreams() {
	if Js == nil {
		utils.Warn("NATS JetStream not available, skipping stream declaration")
		return
	}

	for name, want := range streamSubjects {
		info, err := Js.StreamInfo(name)

		if errors.Is(err, nats.ErrStreamNotFound) {
			// New stream: file storage, so events survive a NATS restart. The
			// externally-created streams are memory-backed; those are left as
			// they are rather than migrated, since storage type cannot be
			// changed on an existing stream.
			if _, addErr := Js.AddStream(&nats.StreamConfig{
				Name:     name,
				Subjects: want,
				Storage:  nats.FileStorage,
				Replicas: 1,
			}); addErr == nil {
				utils.Info("created JetStream stream", "stream", name, "subjects", want)
				continue
			}

			// The API runs multiple replicas, all of which call this at boot, so
			// against a cold NATS they race and every loser sees a "name already
			// in use" style failure. Re-reading settles which it was: if the
			// stream is there now, a sibling created it and we simply carry on to
			// reconcile its subjects.
			info, err = Js.StreamInfo(name)
			if err != nil {
				utils.Error("failed to create JetStream stream", "stream", name, "error", err)
				continue
			}
		} else if err != nil {
			utils.Error("failed to inspect JetStream stream", "stream", name, "error", err)
			continue
		}

		missing := subjectsNotIn(want, info.Config.Subjects)
		if len(missing) == 0 {
			continue
		}

		cfg := info.Config
		cfg.Subjects = append(append([]string{}, info.Config.Subjects...), missing...)
		if _, err := Js.UpdateStream(&cfg); err != nil {
			utils.Error("failed to add subjects to JetStream stream",
				"stream", name, "missing", missing, "error", err)
			continue
		}
		utils.Info("added subjects to JetStream stream", "stream", name, "added", missing)
	}
}

// subjectsNotIn returns the entries of want that have no exact match in have.
// Exact matching is deliberate: a stream carrying "chat.room.closed" does not
// carry "chat.room.closed.*", and treating one as covering the other is the
// mistake that dropped the chat events in the first place.
func subjectsNotIn(want, have []string) []string {
	existing := make(map[string]struct{}, len(have))
	for _, s := range have {
		existing[s] = struct{}{}
	}

	var missing []string
	for _, s := range want {
		if _, ok := existing[s]; !ok {
			missing = append(missing, s)
		}
	}
	return missing
}